Conventionally, an emission type electronic display device includes an electroluminescence display (hereinafter, referred to as an ELD). A constituent element of ELD includes such as an inorganic electroluminescent element and an organic electroluminescent element (hereinafter, also referred to as an organic EL element).
An inorganic electroluminescent element has been utilized as a flat light source, however, requires a high voltage of alternating current to operate an emission element.
On the other hand, an organic electroluminescent element is an element provided with a constitution comprising an emission layer containing a emitting substance being sandwiched with a cathode and an anode, and an exciton is generated by an electron and a positive hole being injected into the emission layer to be recombined, resulting emission utilizing light release (fluorescence and phosphorescence) at the time of deactivation of said exciton; the emission is possible at a voltage of approximately a few to a few tens volts, and an organic electroluminescent element is attracting attention with respect to such as superior viewing angle and high visual recognition due to a self-emission type as well as space saving and portability due to a completely solid element of a thin layer type.
In an organic electroluminescence in view of the future practical application, desired has been development of an organic EL element which efficiently emits at a high luminance with a low electric consumption. Examples of such technologies are a slight amount of a fluorescent substance doped in a stilbene derivative, distyrylarylene derivative or a tristyrylarylene derivative, to achieve improved emission luminance and a prolonged lifetime of an element (for example, refer to Patent Document 1). Further, there are known such as an element having an organic emission layer comprising a 8-hydroxyquinoline aluminum complex as a host compound which is doped with a slight amount of a fluorescent substance (for example, refer to Patent Document 2), and an element having an organic emission layer comprising a 8-hydroxyquinoline aluminum complex as a host compound which is doped with quinacridone type dye (for example, refer to Patent Document 3).
Regarding to the technologies disclosed in the above-described Patent Documents, when emission from an excited singlet is utilized, since a generation ratio of a singlet exciton to a triplet exciton is ⅓, that is, a generation probability of an emitting exciton species is 25% and a light taking out efficiency is approximately 20%, the limit of a quantum efficiency (next) of taking out is said to be 5%.
However, since an organic EL element which utilizes phosphorescence from an excited triplet has been reported from Princeton University (for example, refer to Non-Patent Document 1), researches on materials exhibiting phosphorescence at room temperature have come to be active (for example, refer to Non-Patent Document 2 and Patent Document 4).
Since the upper limit of internal quantum efficiency becomes 100% by utilization of an excited triplet, which is principally 4 times of the case of an excited singlet, it may be possible to achieve almost the same ability as a cooled cathode ray tube to attract attention also for an illumination application. For example, many compounds mainly belonging to heavy metal complexes such as iridium complexes have been synthesized and studied (for example, refer to Non-Patent Document 3).
Further, utilization of tris(2-phenylpyridine)iridium as a dopant has been studied (for example, refer to Non-Patent Document 2). In addition to these, there have been studied to use L2Ir(acac) such as (ppy)2Ir(acac) as a dopant (for example, refer to Non-Patent Document 4). Also there have been studied to use compounds as a dopant, such as tris(2-(p-tolyl)pyridine)iridium (Ir(ptpy)3), tris(benzo[h]quinoline)iridium (Ir(bzq)3) and Ir(bzq)2 CIP(Bu)3 (for example, refer to Non Patent Document 5).
Further, to obtain high emission efficiency, a hole transporting compound is known to use as a host of a phosphorescent compound (for example, refer to Non-Patent Document 6).
Further, various types of electron transporting materials have been used as a host of a phosphorescent compound doped with a new iridium complex for example, refer to Non-Patent Document 4). In addition, a high emission efficiency has been achieved by introduction of a hole block layer (for example, refer to Non-Patent Document 5).
Moreover, there is disclosed materials of an electron transport property having a chemical constitution in which a nitrogen-containing aromatic ring compound extends in two directions or in three directions form a center of a 3 ring type condensed heterocyclic compound (for example, refer to Patent Documents 5, 6 and 7).
Presently, although it is investigated to make further higher efficiency and longer lifetime of the light emission of the organic EL element using this phosphorescence luminescence, and the external extraction efficiency of about 20% which is a theoretical limit was attained about green luminescence, it is only at a low current portion (low luminance area), and the theoretical limit has not been yet attained in a high current region (high luminance region). Furthermore, sufficient efficiency about other luminescent colors has not been obtained, and further improvement is required. A development of the organic EL device which emits light to high-intensity efficiently with low power is desired in the organic EL element towards a future practical application. Especially, regarding to the organic EL element of blue phosphorescence luminescence, it is required that it emits light efficiently with a long lifetime.